"One of my favorite approaches to science is invention, which builds nicely upon a study of simple machines: levers, screws, pulleys, wedges, inclined planes and wheels and axles. Each of this machines can perform work through the exertion of a single force. Knowing these machines is a start, but being able to use them in combination to create complex machines is more of a challenge. And as students begin to create complex machines, they enter the realm of the inventor.

The key measure of a machine’s success is its mechanical advantage: the ratio of the output of force based on the input of force. This is the mathematical expression of how much work is actually being done. Of course when working with simple machines, other forces counteract the work one is trying to do such as friction and gravity. As machines become more complex they are more efficient, doing work while compensating for these forces.

While the wedge, screw and inclined plane get the job done by moving or displacing an object through the use of force, pulleys, levels and wheels and axles operate on the concept of striking a balance to make work easier. In fact, some scientists argue that there are really only five simple machines, categorizing the wedge as nothing more than an inclined plane in motion.

The study of simple machines and the resulting challenge of designing, building and inventing is an engaging unit of instruction that is best termed “hands-on physics.” Through interacting with their environment, students solve problems and develop products that demonstrate an understanding of how these machines work. And the test of their success is the ultimate authentic assessment: does it do the job intended – does it work?

This week’s D12 offers more than a dozen online resources for studying the use of simple machines in doing work and creating more complex machines. I hope you’ll find this collection to be as much fun for you as it is for your students to explore."